A pair of spacewalking astronauts will work on the oldest U.S. solar arrays of the International Space Station on Saturday, taking care to safeguard themselves against the remote chance of electric shocks near the orbital power plant.

NASA astronauts Steven Swanson and Joseph Acaba plan to float outside the space station at about 12:43 p.m. EDT (1643 GMT) to work near the batteries for a pair of 8-year-old solar wings on the outpost’s port-most edge. The chore is one of several to prime the station for future construction and comes one day after Discovery shuttle astronauts unfurled a pair of new solar arrays on the outpost’s starboard side, completing its backbone-like main truss.

But before the spacewalkers exit the station today, they will carefully wrap some of the metal connecting rings on their spacesuits with insulating tape to protect against any slight electrical shocks near their portside worksite, which can be prone to arcing from the surrounding plasma environment.

“With the suits as designed, we believe we have sufficient protection,” space station flight engineer Kwatsi Alibaruho told reporters late Friday.”We’ve just applied some additional factor of safety to drive the probability of a problem absolutely as low as we could.”

Alibaruho said the risk an astronaut receiving even a mild electric shock is extremely remote, and the voltage and currents involved are very small. But NASA rules call for an immediate end to any spacewalk if any shock – no matter how small – should one occur, he added.

Spacewalking astronauts rely on their spacesuits functioning properly, including onboard electronics, while working outside a spacecraft.

Busy spacewalk on tap

Today’s spacewalk is the second of three for Discovery’s 13-day mission, but has been revamped after launch delays prompted NASA cut a planned fourth excursion in order to complete the shuttle flight before the arrival of a Russian Soyuz spacecraft ferrying a new station crew next week.

The spacewalk will be the fourth for Swanson and the first for Acaba, a former schoolteacher who is making his first spaceflight. It will send them to various locations across the space station’s metallic backbone, a massive truss that is longer than a football field.

“They’re ready to go,” Discovery skipper Lee Archambault told Mission Control late Friday. “We’re very much looking forward to another day on orbit.”

Swanson and Acaba will loosen bolts on the portside solar wing batteries so future spacewalkers can replace them later this year. They also plan to prepare the station to receive two future cargo carriers, take infrared photographs of a damaged radiator and install a new navigation antenna to help Japan’s first unmanned cargo ship – the H-2 Transfer Vehicle – dock at the orbiting lab later this year.

While the spacewalkers work outside the station, astronauts inside are expected to begin testing repairs to part of the outpost’s urine recycling system. The device, a distillation assembly, is part of a larger system to recycle condensation, astronaut urine and sweat back into pure water for drinking, food preparation, bathing and other uses. It has been broken since December.

Astronaut Sandra Magnus removed the broken distillation gear on Friday and plans to test its replacement in a dry run later today. If successful, the urine recycler will then be used to purify a batch of water late in Discovery’s mission so new samples can be returned to Earth.

“We have a considerable about of urine in storage containers,” Alibaruho said, adding that the urine is usually discarded aboard disposable Russian cargo ships. “We’ll take some of that …and attempt to process into clean water.”

NASA wants to revive the space urine recycler in order to restore the station’s full recycling system and certify that the water it produces is fit for astronaut consumption.

Recycling water aboard the station is key to plans to boost the outpost’s crew size up to six people later this year. It would allow an extra 15,000 pounds (6,803 kg) of cargo and other supplies, weight that was previously reserved for water deliveries, to be launched to the station, NASA has said.

Discovery and its shuttle astronaut crew launched toward the station on Sunday and are due to land on March 28.